This invention relates to actuators of the type comprising an output shaft which, in use, effects an axial movement, and drive means therefor. In particular the output shaft is intended to effect a controlled, slow moving, extending or retracting axial movement for operating an associated mechanism. A particular application for such actuators is for driving the valve spindle of a valve, where the spindle is extended or retracted in order to force a valve sealing element against the valve seat.
In most existing designs of actuators of the type referred to above, the axial movement of the output shaft is obtained by arranging for a power source to drive the worm of a worm and worm wheel set to effect a single stage reduction in speed, varying typically between 15 to 1 to 80 to 1. The worm wheel rotates a drive nut which is threaded on to the valve spindle so that rotation of the drive nut extends or retracts the output shaft.
There are two applications where this type of actuator introduces installation and operating problems. Firstly, there are applications where the plan area for the equipment is restricted to a relatively small diameter. This occurs in under-sea equipment where the gear box and motor has to be encased in chambers subjected to high external pressures. Also in nuclear reactor engineering and similar sites producing ionising radiation the equipment has to pass through the biological shielding and hence apertures in the shield must be of the smallest possible diameter.
Secondly, there are applications where the equipment must be of the fail-safe type--in the case of a valve, the spindle must move under spring force to one or other terminal position upon failure of the power supply. The conventional actuator is not, normally, suitable for this duty as the drive nut will not normally "back drive" when a force is applied to the spindle.